1. Field of the Invention
The present invention relates to a bearing device and a method for manufacturing the same used for audio appliances, video appliances and business machines.
2. Description of the Prior Art
A bearing device comprising a rotary cylinder for a magnetic head used in audio appliances, video appliances, and the like is known in the art, and typically has a structure as shown in FIG. 1.
In this bearing device, a lower end portion of a shaft member 10, serving as a fixed shaft, is secured to a lower cylinder 30 by pressing the shaft member 10 thereinto. A sleeve 20, which is secured to an upper cylinder 32, is rotatably mounted around the shaft member 10. Herringbone shaped dynamic pressure producing grooves 11a and 11b, for producing a dynamic pressure, are formed in outer surface 15 of the shaft member 10 to form a radial fluid bearing of the dynamic type for supporting the sleeve 20 radially. A thrust receiver 22, comprising a spherical member, is pressed into the inside of the sleeve 20 and secured to an axial upper end portion thereof. An upper end surface 12 (plane) of the shaft member 10 in the axial direction is opposed directly, or indirectly if a lubricant is disposed therebetween, to the thrust receiver 22 to form a thrust bearing of the pivot type for supporting the sleeve 20 axially.
In an upper surface of the upper end of the sleeve 20 is formed an air escape groove 23 extending in the axial direction to create a space extending between the upper end 12 of the shaft member 10 and the thrust receiver 22, to the outside atmosphere. An air escape hole 24 is also formed at an intermediate portion of the sleeve 20, penetrating a side wall thereof in a radial direction, to bring a space 21 extending between the outer surface of the shaft member 10 and the sleeve 20 to the outside atmosphere thereby reducing any air resistance at the time of assembling the bearing device, and to allow air bubbles contained in the lubricant, which may comprise oil or the like, to escape to the outside.
A driving motor for the bearing device includes a cylindrical rotor magnet 34 fixed to a disk 33 which is, in turn, secured to the outer surface of the sleeve 20. A stator coil 35 is fixed to an inner surface of a side wall of the lower cylinder 30. The inner cylindrical surface 25 of the stator coil 35 opposes to the outer surface 26 of the rotor magnet 34 with an air gap 27 therebetween.
Further, a stationary rotary transformer 36 of a cylindrical shape is fixed to the lower cylinder 30 so that the stationary rotary transformer 36 surrounds the peripheral surface of a lower portion of the sleeve 20. A rotating rotary transformer 37 is fixed to the disk 33 such that the cylindrical inner surface 41 of the rotating rotary transformer 37 opposes to the peripheral outer surface 42 of the stationary rotary transformer 36. A signal from a magnetic head 38, fixed to the upper cylinder 32, is transmitted to the stationary rotary transformer 36 through the rotating rotary transformer 37.
The bearing device described above is driven with a lubricant such as oil or grease filled into space 21 of the radial bearing between the sleeve 20 and the shaft member 10. Alternatively, a gas such as air may be used as the lubricant.
When the stator coil 35 of the driving motor is energized, a rotational force is produced in the rotor magnet 34, and the sleeve 20 and each of its accessory parts, secured directly or indirectly to the sleeve 20, are rotated as a unit. As the sleeve 20 rotates, a dynamic pressure is produced by the dynamic pressure producing grooves 11a and 11b of the shaft member 10 to form a fluid film in the space 21 of the radial bearing. As a result, the sleeve 20 is supported radially by the pressure of the fluid film, thus maintaining a non-contacting condition with respect to the shaft member 10. At the same time, since the thrust receiver 22 is brought into point-contact with the upper surface 12 of the shaft member 10, the sleeve 20 is rotated while being supported in the axial direction by the shaft member 10.
In the foregoing bearing device, the thrust receiver 22 generally has a hardness higher than that of the shaft member 10. However, in the prior art bearing device, since the end surface 12 of the shaft member 10 opposing the thrust receiver 22 is a flat plane surface, the end surface 12 of the shaft member 10 is subjected to a large surface pressure at the point of contact with the thrust receiver 22. Thus, a large amount of wear is caused. As a result, the unfavorable situation occurs in which the height of the magnetic head 38, fixed to the upper cylinder 32, decreases gradually while the bearing device is being used.
According to experiments conducted by the inventors of the present application, by using a shaft member 10 of a hardened stainless steel round bar (SUS 420J2) and a thrust receiver 22 of a hardened steel ball (SUJ2), the wear at the end surface 12 of the shaft member 10 amounted to 5 .mu.m or larger after several hundreds of hours use, even when oil is used as the lubricant.
However, in the bearing device of this type, it is required to maintain the change in the height of the magnet head 38, during use, as small as possible. For example, the change in the height is limited to 5 .mu.m or less. Accordingly, the development of a bearing device which meets such a requirement has been needed.